Method and contact point for establishing an electrical connection

ABSTRACT

A method for establishing an electrical connection between a first contact surface and a second contact surface, with a wire-bonding tool being used to provide a contact wire between the contact surfaces by bonding the contact wire to the first contact surface and subsequently leading it to the second contact surface, bonding it to the latter, and subsequently separating it using the wire-bonding tool. After the contact wire has been separated from the second contact surface, the wire-bonding tool is used to provide the contact point with an additional contact securing element via the contact wire.

FIELD OF THE INVENTION

The present invention relates to a method for establishing an electricalconnection and a contact point.

A single-wire contacting method, called bonding, is known forestablishing an electrical connection between at least two contactsurfaces. In doing this, individual wires, in particular, gold oraluminum wires, are positioned between the contact surfaces to bebonded, using a wire-bonding tool. In bonding, the contact wire isbonded to the contact surfaces by applying ultrasonic pressure and heat.The free end of the bonding wire is first melted to form a ball, byapplying thermal energy, and subsequently pressed onto the first contactsurface, using a bonding capillary. The contact wire bonds to thecontact surface as a result of atomic bonding forces (material fusion)arising at the boundary between the contact surface and the contactwire. During bonding to the first contact surface, the ball that waspreviously melted on is deformed into a nail head. The contact wire isthen led to the second contact surface, using the wire-bonding tool. Toprevent the contact wire from breaking away at the first contact point,the contact wire is formed into a loop. The contact wire is then pressedonto the second contact surface with the wire-bonding tool by againapplying ultrasonic pressure and heat. This produces necking of thecontact wire, causing the latter to form a rupture joint at which thecontact wire breaks away from the second contact surface as thewire-bonding tool moves on. The contact wire is bonded to the secondcontact surface by a “stitch,” with atomic bonding forces again arisingat the boundary between the contact wire and the second contact surface.

This known ball-wedge bonding method (ball bonding with the firstcontact surface, and stitch bonding with the second contact surface)produces a strong dependency between the materials of the contact wireand the contact surface, thus forming strong atomic bonding forces atthe boundaries. Particularly when contacting the second contact surface,a relatively weak surface bonding forms between the stitch and thecontact surface, resulting in contacting errors, particularly in thecase of contact surfaces made of hard-to-bond materials.

SUMMARY OF THE INVENTION

The method according to the present invention offers an advantage overthe related art in that it considerably improves the contact stabilityof the bond between the contact wire and the second contact surface. Thefact that the wire-bonding tool provides the contact point with anadditional contact securing element after bonding the second contactsurface increases the contact stability of the second contact point(stitch or wedge) independently of the generation of atomic bondingforce between the contact wire and the second contact surface.

In one preferred embodiment of the present invention, the additionalcontact securing element is provided by the ball shape, applied to thecontact point and subsequently deformed by the bonding tool, at the endof the contact wire that remains free after contacting the secondcontact surface. This makes it possible, after forming the electricalconnection between the contact wire and the second contact surface, toimmediately form the ball on the end of the contact wire that is nowfree and to position it over the contact point as an additional contactsecuring element. A particularly preferred feature is to deform the ballwith the wire-bonding tool so that the contact point overlaps, producingat least one, preferably two, additional bonding areas between theadditional contact securing element and the contact surface. The atomicbonding forces generated cause the additional bonding areas to adhere tothe contact surface, forming a sort of tensile strain relief for thecontact wire bonded to the second contact surface. This very reliablyprevents the contact wire from breaking away from the second contactsurface. The possibility of the contact wire breaking away is nowdetermined only by the rupture strength of the contact wire itself, andno longer by the adhesion between the contact wire and the secondcontact surface, i.e., the contact wire itself breaks before the contactpoint ruptures.

According to another preferred embodiment of the present invention, theproduction of the additional contact securing element can be preciselyreproduced through wire-bonding tool settings, in particular, byprogramming a corresponding controller of the wire-bonding tool. Thismakes it possible to create identical contact securing elements among alarge number of contacts, and these identical contact securing elementscan be easily tested on the basis of a predictable, reproducible result.One particularly preferred feature is that a visual, preferablyautomatic visual, inspection of the contact point is carried out, inwhich the contact securing elements that are not precisely produced,i.e., according to the specified degree of reproducibility, are reliablydetected. This makes it possible to achieve a sort of zero error rate inproducing bonds that result in a higher production yield.

A contact point according to the present invention advantageouslyensures a high contact stability between the contact wire and contactsurface. Since the contact point includes an additional contact securingelement which at least partially engages over the contact wire in thearea of the contact point and forms at least one additional bondingsurface with the contact surface, the available overall surface isadvantageously increased for contacting the contact wire with thecontact surface, enabling the contact point to withstand highermechanical stresses. Particularly when used in safety-relatedcomponents, this contact point can maintain highly redundant electricalconnections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a first schematic view of the process steps inestablishing an electrical connection.

FIG. 1b shows a second schematic view of the process steps inestablishing an electrical connection.

FIG. 1c shows a third schematic view of the process steps inestablishing an electrical connection.

FIG. 1d shows a fourth schematic view of the process steps inestablishing an electrical connection.

FIG. 1e shows a fifth schematic view of the process steps inestablishing an electrical connection.

FIG. 1f shows a sixth schematic view of the process steps inestablishing an electrical connection.

FIG. 1g shows a seventh schematic view of the process steps inestablishing an electrical connection.

FIG. 1h shows a eighth schematic view of the process steps inestablishing an electrical connection.

FIG. 2 shows a first embodiment of a contact point.

FIG. 3 shows a second embodiment of a contact point.

FIG. 4 shows a third embodiment of a contact point.

FIG. 5 shows a fourth embodiment of a contact point.

FIG. 6 shows a fifth embodiment of a contact point.

DETAILED DESCRIPTION

FIG. 1a shows an electrical connection 10 between a first contactsurface 12 and a second contact surface 14. Contact surface 12 isprovided on a substrate 16 and contact surface 14 on a substrate 18.Electrical connection 10 is produced by bonding in the known manner(ball-wedge bonding).To do this, a wire-bonding tool (not illustrated)is used to first heat the free end of a contact wire 20, forming it intoa ball 21. A capillary nozzle of the wire-bonding tool is then used topress this ball 21 onto first contact surface 12, thus producing atomicbonding forces at the boundary between what is then a plasticallydeformed ball 21 and contact surface 12. The wire-bonding tool is thenmoved toward second contact surface 14, thus forming a loop 22 incontact wire 20. Contact wire 20 is pressed with the capillary nozzleonto second contact surface 14, where it is plastically deformed, thusproducing atomic bonding forces between contact wire 20 and secondcontact surface 14. The plastic deformation of contact wire 20 (stitch)by the capillary nozzle simultaneously creates a rupture point at whichcontact wire 20 breaks after the capillary nozzle is removed. The designof second contact point 24 (wedge) has a relatively small contact areabetween contact wire 20 and contact surface 14. As a result, contactpoint 24 allows contact wire 20 to pull away from contact surface 14.Enormous contact problems arise, especially if contact surface 14 ismade of a hard-to-bond material.

Electrical connection 10 illustrated in FIG. 1a is produced by a knownbonding method (ball-wedge method). Such electrical connections 10 areestablished, for example, when microhybrid components are connected tomicrochips.

FIGS. 1b to 1 h illustrate the method according to the present inventionfor establishing electrical connection 10, with this method being basedon an electrical connection previously established according to FIG. 1a.In the following figures, identical components are always identified bythe same reference numbers as in FIG. 1a and are not explained again.

FIG. 1b shows a schematic representation of a capillary nozzle 26 of awire-bonding tool 28. Capillary nozzle 26 has a passage 30 through whichcontact wire 20 is fed. Suitable feed devices enable contact wire 20 tomove through capillary nozzle 26. After electrical connection 10 shownin FIG. 1 a has been established, contact wire 20 moves toward secondcontact surface 14, and its free end 32 is heated to a temperature aboveits melting point, using a thermal energy source. A surface tensioncauses the molten mass of contact wire 20 to form a ball 34. Melting aball 34 onto end 32 takes place directly after establishing theconnection between contact wire 20 and contact surface 14, as shown inFIG. 1a. As a result, it is not necessary to reposition capillary nozzle26 in relation to contact point 24.

According to the next process step illustrated in FIG. 1c, a force andultrasound are applied to capillary nozzle 26. This compresses ball 34,which undergoes plastic deformation. The shape of capillary nozzle 26can influence the plastic deformation of ball 34. In the illustratedembodiment, the end of capillary nozzle 25 facing contact surface 14 hasa circumferential ring-shaped ridge 36 that engages with an inner cone38.

The plastic deformation of ball 34 follows the shape of this inner cone38. The application of contact force F, combined with ultrasound energy,produces atomic bonding forces between ball 34 and, extending fromcenter 40 of bonding point 24 to the wedge, causing deformed ball 34adhere to contact point 24 during a motion away from contact surface 14,as shown in FIG. 1d.

In a subsequent process step, illustrated in FIG. 1e, capillary nozzle26 is moved laterally away from contact point 24. This movement isindicated by an arrow 40. If necessary, lateral movement 40 can besuperimposed on the lifting of capillary nozzle 26 away from contactpoint 24 (FIG. 1d). Movement 40 is oriented so that its direction vectoris more or less contrary to a longitudinal extension of contact wire 20laid in loop 22. Direction of movement 40 is maintained until a vertex44 of ring-shaped ridge 26 of capillary nozzle 26 has passed animaginary perpendicular running through deformed ball 34 (perpendicularthat is parallel to the axis of capillary nozzle 26). As shown in FIG.1f, capillary nozzle 26 is moved in the direction of contact surface 14so that ring-shaped ridge 36 strikes plastically deformed ball 34.Depending on contact force F′ applied, and under the influence ofultrasound, ball 34 undergoes a further plastic deformation, due to theouter lateral surface of ring-shaped ridge 36. Ball 34 undergoes asegment-like deformation. Ball 34 continues to change shape untilsegment 34′ projects laterally over contact wire 20 already bonded tocontact surface 14 and comes into physical contact with contact surface14 in additional bonding areas 48. This type of deformation generatesatomic bonding forces between segment 34′ formed and contact surface 14,causes the segment to permanently adhere to the surface. After capillarynozzle 26 moves away, segment 34 spans contact wire 20 in the area ofcontact point 24 and holds the latter in place like a strap, as shown inFIG. 1g. Segment 34 provides a kind of tensile strain relief function tosecure contact point 24 for contact wire 20. The remaining wire thatbroke away when capillary nozzle 26 was removed is visible in center 40of contact point 24 in the form of a pointed elevation 50. The latterforms an additional positive-lock joint with contact wire 20 in the areaof contact point 24.

FIG. 1h shows an enlarged representation of contact point 24 afterelectrical connection 10 has been established with additional contactsecuring provided by segment 34′. It is clear that segment 34′ hascertain shape characteristics that are derived from the size of ball 34(FIG. 1b), the size and type of bonding parameters (FIG. 1f), and theshape of ring-shaped ridge 36 of capillary nozzle 26. Because the shapeof capillary nozzle 26, the magnitude of bonding forces, and the size ofball 34 are known or can be set, segments 34′ can be achieved inreproducible shapes. After contact point 24 has been produced, segment34′ can be measured by an optical monitoring unit (not illustrated).Comparing the measured shape of segment 34′ to a previously stored shapemakes it possible to draw conclusions about the quality of contact point24. If the shape of segment 34′ matches the expected shape, a perfect,i.e., contact-secure and additionally secured contact point 24 can beassumed, making it possible to produce contact point 24 with zeroerrors. Fault-free usage values can thus be predicted, particularly whenusing electrical connections on microchips in safety/security systems.

The present invention is, of course, not limited to the embodimentillustrated in FIG. 1. In particular, different shapes can be selectedfor segment 34′. FIGS. 2 through 6 show different embodiments of segment34′. The shape of segment 34′ can be selected, for example, by choosinga different shape for capillary nozzle 26 and varying the placement ofcapillary nozzle 26 when shaping ball 34 into segment 34′. In addition,the design of segment 34, and the way it bonds to contact surface 14,can be influenced by setting general bonding parameters, such as force For the frequency and intensity of the ultrasound energy.

According to further exemplary embodiments, it is possible, inparticular, to provide a more shallow depression between tip 50 andsegment 34′. This means that the transition between segment 34′ andpoint 50 occurs through a relatively shallow depression, therebyimproving the positive-lock joint between contact wire 20 and segment34′ or contact point 24 in the example.

By way of example, FIG. 2 shows a segment 34′ that is designed in theshape of a ridge. In contrast to this, segment 34′ in FIG. 3 has aflatter design and merges with the material of tip 50. According to theembodiment illustrated in FIG. 4, segment 34′ has an even flatterdesign, so that it is almost shaped like a disk and also merges with thematerial of tip 50. FIG. 5 shows a further embodiment, in which segment34′ is shaped like a shallow basin, with segment 34′ again merging withthe material of tip 50. Finally, FIG. 6 shows an embodiment of segment34′ in which segment 34′ is designed as a largely flat disk that has aridge-shaped bulge in the direction of contact wire 20. Tip 50, in thiscase, is formed from the material of segment 34′ by deforming ball 34accordingly. This can be achieved by a suitable design of capillarynozzle 26 and placement of capillary nozzle 26 while forming the ballinto segment 34′.

It is also possible to modulate segment 34′ as a largely rectangularobject having a defined elongation in the x-direction, y-direction, andz-direction by setting the bonding parameters and/or placementparameters of capillary nozzle 26 while shaping segment 34′. Parametersthat can be set while shaping ball 34 into “rectangular” segment 34′make it possible to set precisely reproducible dimensions in the x-, y-,and z-directions. A subsequent visual, in particular automatic visual,inspection of contact point 24 can be used to easily and effectivelycheck contact point 24 for freedom from errors.

What is claimed is:
 1. A method for producing an electrical connectionbetween a first contact surface and a second contact surface, comprisingthe steps of: in accordance with an operation of a wire-bonding tool,placing a contact wire between the first contact surface and the secondcontact surface by performing the steps of: bonding the contact wire tothe first contact surface, subsequently leading the contact wire to thesecond contact surface, bonding the contact wire to the second contactsurface, and subsequently separating the contact wire with thewire-bonding tool; separating the contact wire from the second contactsurface; after separating the contact wire from the second contactsurface and in accordance with the operation of the wire-bonding tool,providing a contact point with an additional contact securing elementvia the contact wire; providing the additional contact securing elementby performing the steps of: applying a ball to the contact point, and inaccordance with the operation of the wire-bonding tool, subsequentlydeforming the ball at an end of the contact wire that is free afterbonding the second contact surface; following a plastic predeformationof the ball, laterally moving a capillary nozzle of the wire-bondingtool away from the contact point in accordance with a direction vectorthat is contrary to a longitudinal extension of the contact wire betweenthe first contact surface and the second contact surface; andsubsequently moving the capillary nozzle to the ball to form theadditional contact securing element and provide a final plasticdeformation to the ball.
 2. The method according to claim 1, wherein thestep of deforming the ball includes the step of: deforming the ball withthe wire-bonding tool into the additional contact securing element sothat the additional contact securing element overlaps the contact pointand forms at least one additional bonding area with the second contactsurface.
 3. The method according to claim 1, further comprising the stepof: heating the end of the contact wire to a temperature above a meltingpoint of the contact wire in order to form the ball.
 4. The methodaccording to claim 1, further comprising the step of: first predeformingthe ball plastically in a center of the contact point.
 5. The methodaccording to claim 1, further comprising the step of: maintaining themovement of the capillary nozzle until a vortex of a ring-shaped ridgeof the capillary nozzle has passed an imaginary perpendicular runningthrough the predeformed ball parallel to an axis of the capillarynozzle.
 6. The method according to claim 1, wherein: a shape of theadditional contact securing element is capable of being reproducibly setby programming the wire-bonding tool.
 7. The method according to claim6, wherein: the shape of the additional contact securing element iscapable of being reproducibly selected by setting at least one ofbonding parameters and placement parameters of the wire-bonding tool. 8.The method according to claim 1, further comprising the step of:visually inspecting the additional contact securing element after thewire-bonding tool is removed.
 9. The method according to claim 1,wherein: the step of visually inspecting is performed automatically.